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Old 14th December 2010, 04:00 AM   #141
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Hmmm, no one stepped up with a possible solution...? I'm deeply disappointed in you all out there in TV land.

I went ahead and figured it out myself. It is the power supply I added to the main boards that power the pot board. I tapped off the raw supply which was a completely stupid thing to do. I swear this project is cursed!
Despite 2 regulators in series, the 5V supply for the digital pot had HUGE ripple - enough to indeed trigger the INC pin and the U/D pin on the IC - yes, that was the wiper going up and down.
Now I need to reconfigure the main board to pull the voltage from a clean source, such as the reference voltage. I will NOT be making new boards to fix this though.
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Old 14th December 2010, 10:13 AM   #142
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And there are Members here recommending that Beginners and more experienced builders design a power amplifier with a regulated power supply !
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Old 14th December 2010, 03:57 PM   #143
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Quote:
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And there are Members here recommending that Beginners and more experienced builders design a power amplifier with a regulated power supply !
Well, this is a bit more complex, due to the variable voltage and current limiter. Throw another wrench in the works with a digital pot, and there are bound to be details that get overlooked.
Remove the current limiter and the pot that controls the voltage output and this would be a very good regulator for an amp. Stable voltage, regardless of the load (within reason) and no significant output ripple.
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Old 14th December 2010, 04:03 PM   #144
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Hi MJL,
I'm not suggesting your PSU could or could not drive a power amplifier.

What I am reminding other folk about is the false idea that a regulated PSU is easy and can be recommended to any power amplifier builder as an easy "Sound Quality Improver".

You are showing us that a good PSU is not at all easy to get right. You are using your skills to get it right.
It is not simple to integrate a Power Amp and Regulated PSU.
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Old 14th December 2010, 06:04 PM   #145
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Originally Posted by MJL21193 View Post
I went ahead and figured it out myself. It is the power supply I added...
No actually. I went to the considerable trouble redoing the 5V supply for this only to see absolutely no difference.
So, extreme measures:
I disconnected it from the main board altogether and ran it from a 9V battery with a 7805 reg - guess what? SAME result, that nonsense waveform at the wiper output (as seen in post #140).
Either the chip is damaged or it is somehow oscillating, even though the datasheet makes no mention of oscillation as a problem.
It is very frustrating - I could have made a very reliable mechanical coupling to a trimpot by now and be done with it.
As much as I hate giving up on something, I need to cut my losses and set aside the digital pot idea. Another example of not thoroughly prototyping something before using it.
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Old 14th December 2010, 06:39 PM   #146
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I could have made a very reliable mechanical coupling to a trimpot by now and be done with it.
The best approach is to not make assumptions. I assumed that a multiturn pot was the only viable option for the voltage control and that assumption now turns out to be false. I have connected a regular single turn pot and it is fine - I can get the voltage adjusted very quickly and accurately.
All that hassle and frustration (and money) for something that could have been as quick and easy as the current control pot.
Live and (maybe) learn.
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Old 15th December 2010, 01:49 AM   #147
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The fix:

NPX_01091.JPG

The new pots mounted for the voltage control. Unfortunately, the ideal place to put them was between the holes that the switches were in.
Photo shows supply 'A' drawing 2.2A at 35VDC and that 2.2A is the max current possible. I won't bother to show the scope at this output - you've seen that flat line before

A few minor things to do 'under the hood' and it will be done. I need to make a bezel (probably from thin hardwood) to cover the switch holes in the front panel and it will have some graduations for the voltage knobs and current limit adjust knobs.

Last edited by MJL21193; 15th December 2010 at 01:55 AM.
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Old 15th December 2010, 01:07 PM   #148
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What about coarse and fine adjustment of the output voltage.
Coarse capable of going through the whole range and fine covering only 5% to 10% of the full range?
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Old 15th December 2010, 11:41 PM   #149
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Quote:
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You are using your skills to get it right.
More like pure bull-headed determination

Quote:
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What about coarse and fine adjustment of the output voltage.
Now THIS is an excellent idea! If only I had my time back, I'd have arranged the front panel to fit 2 more pots in. That would have been a better solution than a digital pot and nearly as good as a multi-turn.
I'll see if I can mount them on the 'cow catcher'
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Old 16th December 2010, 09:14 AM   #150
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The coarse could be switched rather than continuous.
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